Microlevelling - Extract Noise

Use the Microlevelling > Extract Noise menu option (DECORR GX) to calculate the line-to-line noise and save it to the database.

PGW Microlevelling 3.0 - Extract Noise dialog options

X channel

Select the X channel from your active database.

The default is "X".

Script Parameters: MICLEV.XCHAN

Y channel

Select the Y channel from your active database.

The default is "Y".

Script Parameters: MICLEV.YCHAN

Data channel to level

Select the data channel to level.

Script Parameters: MICLEV.OCHAN

Name for decorrugation noise channel

Provide the name of the decorrugation channel in which to save the calculated noise.

Script Parameters: MICLEV.NCHAN

Flight line spacing (in same units as X and Y)

Specify the flight line spacing in the same units as the X and Y units.

Script Parameters: MICLEV.SPACE

Line direction - degrees azimuth (c/w from N)

Specify the flight line direction in degrees azimuth, clockwise from vertical on the screen.

Script Parameters: MICLEV.DIR

Cell size for gridding

Specify the grid cell size to use for gridding.

The default is 1/5th of the line spacing.

Script Parameters: MICLEV.CELL

Decorrugation cutoff wavelength

Specify the decorrugation high-pass wavelength.

The default is four times the line spacing.

Script Parameters: MICLEV.WLEN

Name of original grid file, new or existing

The input channel is gridded and saved as a grid. The first time through, the input channel is gridded. In subsequent iterations, you can provide the previously gridded file.

Script Parameters: MICLEV.OGRID

Name of decorrugation noise grid file

Specify the name of the output noise grid file. The directional noise is calculated on a 2D grid then sampled to the database. If the file already exists, you are prompted to either overwrite it or exit.

Script Parameters: MICLEV.NNOISE

[Advanced>]

Click the Advanced button to adjust the minimum curvature gridding parameters.

  • For more information on the 'Gridding - Advanced Options' dialog parameters, refer to the Minimum Curvature Gridding topic.
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    Application Notes

    Microlevelling is applied after tie line levelling to remove any residual low-amplitude, directional line-to-line noise that was not removed by the tie line levelling process. Since microlevelling applies a directional short wavelength filter, it does not alter the response of the geology, which is not directional and contains longer and higher amplitude wavelengths.

    Extract Noise applies a directional high-pass filter on the gridded input channel in the direction perpendicular to the flight lines and produces the decorrugation noise grid. (If the file already exists, you will be prompted to either overwrite it or exit.) It then samples it to the database. Microlevel subsequently subtracts the calculated noise from the original data and saves a levelled channel, from which a final levelled grid may be generated.

    If the input grid already exists, specify it as the “original grid” and refrain from overwriting it. The first time through, however, the input channel will be gridded using Minimum Curvature Gridding and saved under the specified “original grid “ name. If necessary, click the Advanced button to customize the minimum curvature gridding parameters.

    The default grid cell size is 1/5th of the line spacing, which can be modified if desired. A smaller cell size will give a more accurate result, but a larger cell size will make the dialog run faster. The noise channel contains the line level drift component of the data, but it may also contain some residual high-frequency components of the geological signal.

    Microlevel applies amplitude limiting and low-pass filtering to the noise channel to remove the residual geological signal and leave only the component of line level drift, which is then subtracted from the original data to produce a levelled output channel (default name is "miclev").

    Extract Noise calculates default amplitude limit and filter length values for use in microlevelling, but the skilled user may be able to set better values for these parameters based on an inspection of the noise grid. (The microlevelling process is broken up into two separate dialogs to enable you to do this.) Flight line noise should appear in the decorrugation noise grid as long stripes in the flight line direction, whereas geological anomalies should appear as small spots and cross-cutting lineaments, generally with a higher amplitude than the flight line noise but with a shorter wavelength in the flight line direction. You can estimate the maximum amplitude of the flight line noise and set the noise amplitude limit value accordingly. Similarly, you can estimate the minimum wavelength of the level drift along the flight lines and set the low-pass Naudy filter width to half this wavelength. The defaults are to set the amplitude limit equal to the standard deviation of the noise grid and to set the filter width equal to five times the flight line spacing.

    There are two types of amplitude limiting modes that can be applied: clip and zero. In clip mode, any value outside the limit is set equal to the limit value. In zero mode, any value outside the limit is set equal to zero. The clip mode makes more sense intuitively, but it has been found in practice that the zero mode may reject geological signals better, depending on the particular data set. As a rule, the zero mode works better on data sets in which the noise grid contains a lot of high-amplitude geological signals (e.g., shallow basement areas). For data sets in which the noise grid contains mainly flight line noise (e.g., sedimentary basins), the clip mode works better.

    Microlevelling applies a level correction to the traverse lines only. If it is desired to grid the tie lines together with the microlevelled traverse lines, then it may be necessary to also apply a level correction to the tie lines so that their values agree with the microlevelled traverse lines at the intersections. This may be done as follows:

    1. Copy the tie line values to the microlevelled channel.
    2. Use the Intersections menu option to find cross-difference values for the microlevelled data.
    3. Use Load Correction menu option to load these cross-difference values to the tie lines.
    4. Apply the Full Level menu option to the tie lines. The output will be a set of tie lines that matches the microlevelled traverse lines at all intersections.
    5. Copy the microlevelled traverse line values into the same channel as the corrected tie line values.
    Decorrugation Filter

    The decorrugation tool applies two 2D FFT filters:

    1. A Butterworth High-Pass filter with a power of 6 and a cutoff wavelength of 4 line spacings. It is recommended to adjust the wavelength and the power for optimal output.

    2. A Directional Cosine filter that rejects short wavelengths in the direction normal to the survey lines. The default order of the filter is 2.

      The filter is applied in the direction perpendicular to the line direction provided in the dialog.

    The equivalent magmap filter will be:

    BTWR   1.0/4xLine_spacing   6   0 /

    DCOS    Line_direction+90      2   1 /

    See Also: